1. Field of the Invention
The present invention relates to a tilt control device and method for correcting tilt of a recording surface of an optical disc, such as, for example, a recordable or read-only disk, by using a multi-dimensional actuator, e.g., a 3D actuator.
In recent years, optical disc apparatuses have been developed for recording and reproducing large quantities of data.
Principally, an optical disc should be kept in a flat disc shape when it is set on a disc motor, so that an optical pickup unit can keep its optical axis perpendicular to the recording surface of the disc during recording and reproducing operations. For scanning the recording tracks, the optical pickup unit moves in a radial direction in alignment with the radius of the optical disc.
However, the optical disc set on the disc motor is not flat, mainly due to the manufacturing process. The optical disc curves both in the radial and circumferential directions. Furthermore, all kinds of drive tolerances may be faced. Therefore, the optical pickup unit cannot scan the recording tracks with its optical axis perpendicular to the recording surface of the disc. Furthermore, the angle varies according to the position of the optical pickup unit with respect to the optical disc. The angle formed between the optical axis and the recording surface in a radial direction is defined as the radial tilt angle. Additionally, the angle formed between the optical axis and a line tangential to the recording track (or perpendicular to the radius) of the optical disc is referred to as the tangential tilt. Generally, the amount of radial tilt of the optical disc is within a range of ±10 mrad
The user data is extracted from the HF signal. Due to, e.g., time errors in the HF signal, a certain amount of jitter is always present when reading out an optical disc. Some contributors for such a jitter are intersymbol interference, crosstalk between neighboring tracks and disc manufacturing imperfections, but also ordinary noise, present in all electrical circuits. The tilt angle between the disc and the objective lens, is a result of two major contributors, namely, the disc (contribution by manufacturing tolerances and environmental changes) and the drive (contribution by objective lens actuator, turntable motor adjustment, axis adjustment, etc.). The resulting angular deviations lead to comatic aberrations, i.e., a distortion of the optical readout spot on the disc. This distorted readout spot directly results in a distorted HF signal and thus in timing errors, i.e., jitter. Generally, the jitter increases at a greater rate as the radial tilt becomes larger.
Tighter system tolerances in systems like DVD and DVD+RW require decreased maximal allowed tilt errors. These maximal allowed tilt errors are specified in a tilt window which can be, e.g., ±8 mrad in the radial direction. This tilt window is defined to achieve a jitter below a certain required level (typically 15%). If the total tilt in the readout system is larger than this window, the jitter will be too high and active tilt compensation is required. Therefore, in order to reproduce the original signal from the recording track under such fluctuation factors, it is necessary to adjust the optical disc apparatus with respect to the physical fluctuations, such as radial tilt and/or tangential tilt.
2. Description of the Related Art
Japanese Patent Publication No. JP-A-2000-195080 discloses a conventional optical disc apparatus in which a tilt control is applied. In operation, tilt detection means outputs a tilting signal having a voltage corresponding to the tilting amount of the optical disc with respect to an optical head used for recording and reproducing. The tilt control means drives the tilt actuating means or radial tilt adjuster such that a tilt control signal becomes zero. Thus, the optical head can be kept in a position parallel to the optical disc so that a signal with a good quality can be recorded thereto or reproduced therefrom.
Furthermore, International Patent Application No. WO-A-00/16321, corresponding to U.S. Pat. No. 6,157,600, discloses a tilt control device and method, wherein a sledge of an optical pickup unit is placed on a tilting unit which can tilt or incline in the radial direction with respect to the optical disc. In response to a change in the radial tilting angle, the angle of the optical axis of the pickup unit changes with respect to the recording surface of the disc. As a result, the jitter of the signal reproduced also varies.
The tilt sensor may be calibrated on the basis of a jitter measurement in a predetermined calibration data track area. In response to a change in the radial tilting angle, the angle of the optical axis of the pickup unit, with respect to the recording surface of the disc, also changes. As a result, the jitter of the signal reproduced from the calibration data track also changes. The position at which the jitter of the reproduced signal shows its smallest value is the optimum position for the radial tilt. However, a sensor calibration based on the jitter is not possible for non-written optical discs, such as DVD+RW discs.
To reduce costs, the tilt sensor can be omitted. The tilt measurement may then be based on the dz/dr method, wherein the DC components of the radial tilt are measured using the actuator of the optical reading system. In particular, the DC radial tilt is measured through differentiation of the DC actuator voltage, i.e., the mean actuator voltage over one disc revolution, to the radius r, times the DC sensitivity of the actuator. However, this tilt measurement is very sensitive to the focus DC sensitivity and, therefore, has to be calibrated. Such a calibration can easily be performed in drives with a tilting frame. Unfortunately, this calibration is not possible for drives that use a 3D actuator to compensate tilt.